Oxidation of para-substituted phenols was suggested as a method for commercial production of hydroquinone (Japanese Patent Publication No. 36739/1976). The present inventors previously suggested an improved method in which p-isopropenyl phenol in a slightly or hardly water-soluble high-boiling polar solvent is used as a starting material in the above-mentioned method (Japanese Laid-Open Patent Publication No. 106929/1975). On further investigation, the present inventors found that in order to practice this method commercially, it is very important to control the oxidation reaction temperature properly. The specification of the above-cited Japanese Patent Publication No. 36739/1976 states that the starting p-isopropenyl phenol is mixed with a peroxide at about 20.degree. C., a strong acid is added to this mixture, and the resulting mixture is maintained at a temperature in the range of about 10.degree. C. to about 40.degree. C. to perform the reaction. However, the present inventors energetically and detailedly studied the effect of the oxidation reaction temperature on the reaction, and found that according to such a temperature control, the peroxide of the starting p-isopropenyl phenol sometimes builds up in the reaction system, and abruptly decomposes to induce a vigorous exothermic reaction, thus causing troubles to the commercial practice of the reaction. It was also found that when as shown in specification of the above-cited Japanese Patent Publication No. 36739/1976, the mole ratio of the peroxide to the starting p-isopropenyl phenol is 1 to 1.5, adverse effects are exerted on the yield and purity of hydroquinone. The present inventors also discovered that as the catalyst for the oxidation reaction, a strong acid having a pKa value of not more than about 1.0 is used as disclosed in the above-cited Japanese Patent Publication, but depending upon its concentration, it may adversely affect the yield and purity of hydroquinone, and the formation of a hydroperoxide of p-isopropenyl phenol during the reaction. Furthermore, the reaction mixture left after substantial termination of the reaction still contains the unreacted oxidizing agent and the hydroperoxide of p-isopropenyl phenol, a reaction intermediate. If these compounds are left as they are, they will react with the resulting hydroquinone to form impurities such as benzoquinone, and cause a reduction in the yield of hydroquinone.
Extensive investigations of the present inventors in an attempt to solve these problems have now led to the accomplishment of the present invention.